With the ultimate goal of creating an AIDS vaccine that elicits CD8+ T lymphocyte (CD8-TL) responses, it will be important to understand the complete virus-specific response. Despite many years of effort, it is becoming clear that the full extent of the CD8-TL response against AIDS viruses is not well understood. We have obtained exciting new data that indicate that a vaccine that solely elicits cellular immune responses can control acute phase virus replication of a heterologous challenge. We also recently discovered that CD8-TL responses directed against a cryptic epitope, derived from an alternate reading frame of the env gene, was potent at restricting virus replication and selecting for viral escape. We have since found that recognition of cryptic epitopes is a common occurrence in SIV-infection. Based on these exciting data, we hypothesize that including cORFs in a vaccine regimen will increase the breadth of the elicited immune response and will enhance viral control upon infection. Here, we propose a series of experiments that will shed light on the role cryptic epitope-specific responses play in controlling AIDS virus-replication. In the R21 phase, we will examine the total extent of cryptic epitope-specific responses by scanning SIV-infected macaques for responses to overlapping peptides spanning all cryptic ORFs in the SIVmac239 genome. In the R33 phase, we will vaccinate macaques with a DNA prime, attenuated Ad5 boost expressing cryptic ORFs deemed immunogenic in the R21 phase, either with or without known classical SIV ORFs. These animals will be challenged with SIVsmE660 and immune responses and viral control measured. Together, these experiments will lead to a better understanding of the AIDS virus-specific cellular immune response and might lead to novel targets for a T cell based vaccine for HIV. PUBLIC HEALTH RELEVANCE: The recent failure of the Merck Step Trial underscores the need to understand the complete immune response against AIDS viruses. This project will lead to a greater understanding of these responses and might lead to the discovery of new vaccine targets.